The present invention relates to a transmission system using a wet type multi-plate clutch for shifting or as a main clutch, wherein the power train is adopted in working vehicles particularly such as tractors and the like.
In working vehicles such as tractors and the like or ordinary vehicles, a dry type clutch to be pressed against a flywheel of an engine is generally used as a main clutch, which is engaged/disengaged between an engine and a shift mechanism and is half-clutched for generating inching during a transition period between states of engaging and disengaging. In many cases, a dry type multi-plate clutch is used so as to secure transmission torque while being engaged and frictional resistance while being half-clutched. However, since the multi-plate clutch is complex and axially elongated, if possible, a single-plate clutch being simple and compact is desired.
In some cases, a wet type (hydraulic type) multi-plate clutch may be used as the above-mentioned main clutch. However, it has the problem that, while the clutch being disengaged, power is transmitted slightly due to the viscosity of lubricant oil therein so as to generate a creep phenomenon. To protect this problem, it has been considered that a brake is provided in a power train located on the downstream of the wet type multi-plate clutch to brake the downstream power train, and actually this idea becomes known. When such a brake is operated, the downstream side of the clutch is isolated with power transmission, however, the power train on the upstream side of the clutch is still rotated following the engine power. Therefore, the load resulting from the viscosity of lubricant oil in the wet type multi-plate clutch is exerted on the power train on the upstream side of the clutch. The load is further put on the engine at idle, thereby causing incomplete combustion at worst so as to generate the problem that a carbon deposit is accumulated in an engine cylinder.
Then, it is considerable that the creep phenomenon, i.e., a drag of shift mechanism and the like on the downstream side of the disengaged wet type multi-plate clutch resulting from the viscosity of lubricant oil of the wet type multi-plate clutch is prevented by using a conventional dry type clutch whose clutch disc is pressed against the flywheel on the upstream side of the wet type multi-plate clutch. In this case, the dry type clutch is disengaged to isolate only the dragging driving power train with transmission of engine power. Therefore, the dry type clutch can be constituted by a single-plate type clutch, which is simple and compact. This clutch is detached from the flywheel to be disengaged, thereby greatly reducing load on the engine at idle so as to effect an engine protection.
Further, conventionally, there is a well-known working vehicle particularly such as a tractor or the like, which is provided with a shift mechanism including a plurality of parallel power trains with different reduction ratios, wherein the power trains are provided with respective wet type multi-plate clutches. In these shift mechanisms, there is a well-known shift mechanism in which one of the plurality of wet type clutches is selectively engaged and one corresponding speed stage is formed through the power train having the engaged clutch. These wet type multi-plate clutches for shifting are, as they are, used also as a main clutch when all the wet type multi-type clutches are disengaged so as to isolate the transmission system with transmission of engine power. In this case, it is not unnecessary to provide the foregoing wet type multi-plate clutch as a main clutch separately. In this case, the foregoing detriment of the drag inherent to a wet type multi-plate clutch can be dissolved by providing a drag-preventing clutch such as a dry type single-plate clutch which is arranged on the upstream side of the shift mechanism as described above and pressed against the flywheel. Also, therefore, a simple and compact transmission system can be configured.
However, when the wet type multi-plate clutches used for shifting is diverted to a main clutch as they are, each wet type multi-plate clutch possibly come to be half-clutched because a shift operation (operation selecting speed stages) and a main clutch operation are performed separately from each other. To generate a state of half-clutch, each clutch is required to have the larger volume to secure the durability against a load. However, the whole size of shift mechanism becomes significantly large if all the wet type multi-plate clutches used in the shift mechanism have so large volume. Then, it is thought of that only one wet type multi-plate clutch in the shift mechanism has a sufficiently larger volume and a speed stage corresponding to this clutch is selected for the main clutch operation. However, it is too burdensome to be required for such a shift operation in every main clutch operation.
Furthermore, there is the problem associated with the shift operation in such a shift mechanism comprising a plurality of wet type multi-plate clutches that, during the shifting, the fluctuation of hydraulic pressure arises necessarily because the shifting is attended with such a control that hydraulic oil is drained from the clutch previously supplied with the hydraulic oil and the hydraulic oil is supplied to another clutch. When the drainage of hydraulic oil from the clutch and the supply to the other clutch is performed rapidly or the timings of the drainage and the supply are apart, the fluctuation becomes rapid and impact is developed.
A first object of the present invention is to provide a transmission system for a vehicle, wherein, while a wet type multi-plate clutch having high transmission torque and high frictional resistance during its half-clutching is used as a main clutch, a creep phenomenon does not occur when the transmission system is isolated with transmission of engine power and the power trains thereof may be secured to be compact.
According to a vehicle transmission system of the present invention, a plurality of wet type multi-plate clutches for shifting, among which one clutch is selectively engaged to form one speed stage, is utilized as a main clutch. All the wet type multi-plate clutches for shifting are disengaged when the transmission system is isolated with transmission of engine power. Furthermore, a dry type single-plate clutch is provided as a drag-preventing mechanism on the upstream side of the shift mechanism in the transmission system. The dry type single-plate clutch is disengaged when all the wet type multi-plate clutches for shifting are disengaged.
One wet type multi-plate clutch to be half-clutched for generating inching during a transition period of a main clutch between its disengaging and engaging is specified among the plurality of wet type multi-plate clutches for shifting. Only the specified clutch has the largest volume so as to resist against frequent use, thereby securing the whole shift mechanism to be compact. A clutch corresponding to the lowest speed stage in the shift mechanism is used as the clutch for inching.
In this structure, the vehicle transmission system is hydraulically controlled so that, when the main clutch operation is performed while one speed stage corresponding to one wet type multi-plate clutch other than the clutch for inching is determined, only the clutch for inching is half-clutched in the shift mechanism in a transition period between clutching-on and clutching-off during the main clutch operation regardless of the determined speed stage. Therefore, the selected clutch for shifting other than the clutch for inching is prevented from being half-clutched.
The plurality of wet type clutches for shifting in the shift mechanism are engaged with the supply of hydraulic oil and disengaged with the drainage of hydraulic oil.
The drag-preventing clutch apparatus is disengaged and completes engaging while the plurality of wet type clutches play up to a rise of minimum hydraulic fluid pressure therein. In this regard, the drag-preventing clutch apparatus is half-clutched when the wet type clutches play, thereby reducing friction load generated thereon during its half-clutching so as to secure the durability of the drag-preventing clutch apparatus. Also, the main clutch operation between its clutch-on and clutch-off states is performed smoothly.
The drag-preventing clutch apparatus and the shift mechanism are contained in one housing so as to configure a compact portion of the vehicle transmission system for both main-clutching and shifting. Further, by locating an actuator for the drag-preventing clutch apparatus above the outside of the housing, the housing becomes compact and it becomes easy to assemble and maintain the actuator and a link mechanism between the actuator and the drag-preventing clutch apparatus.
Furthermore, the actuator for the drag-preventing clutch apparatus is hydraulically controlled, whereby the hydraulic oil, which is an operating power source of the wet type multi-plate clutches for shifting, may be used also as an operating power source of the actuator. Further, by incorporating control means for supplying and drainage the hydraulic oil in a hydraulic control mechanism of the wet type multi-plate clutches, a hydraulic oil circuit for the actuator may be easily configured to be compact in association with the hydraulic control mechanism for shifting.
Main clutch operation means for switching transmission and isolation of engine power to and from said power train, respectively, is provided. The full range of stroke of the main clutch operation means is divided into a main-clutch-on range where one of the wet type clutches for shifting corresponding to a determined speed stage is selectively engaged, an inching range where the clutch for inching is half-clutched, and a main-clutch-off range where all the wet type clutches for shifting are disengaged. The drag-preventing clutch apparatus is disengaged and completes engaging while the main clutch operation means is located in the main-clutch-off range. The engaging hydraulic pressure of the clutch for inching increases as the stroke of the main clutch operation means within the inching range is varied from the main-clutch-off range towards the main-clutch-on range.
In such a structure, an oil path switching valve is provided as one of hydraulic control devices controlling a flow path of hydraulic oil for each wet type clutch for shifting of the shift mechanism. In the case that one wet type clutch to be selected from the wet type clutches and engaged based on a determined speed range setting of the shift mechanism is a clutch other than the clutch for inching, when the main clutch operation means is located in either the main-clutch-off range and the inching range, the oil path switching valve connects a supply path of hydraulic oil to the clutch for inching, and connects a drain oil path to the wet type clutch for shifting selected from the wet type clutches based on the determined speed stage, which should be engaged essentially. BY such a structure of hydraulic control, while a speed stage corresponding to one wet type clutch other than the clutch for inching is determined, it is not required to shift to the speed stage corresponding to the clutch for inching once during the main clutch operation. The clutch for inching is automatically half-clutched and the other wet type clutches is disengaged during a transition of a main clutch between its engaging and disengaging states. Accordingly, it is possible to provide a main clutch mechanism which generates a state of half-clutch in only the specific clutch for inching without a burdensome operation.
The oil path switching valve is made of an electromagnetic solenoid which is switched based on the detection of a position of shift operation means for determining a speed stage of the shift mechanism and on the detection of a position of the main clutch operation means. Therefore, the hydraulic control for making the clutch for inching half-clutched may be performed at the appropriate times by electric power through simple configuration.
Further, a border between the inching range and the main-clutch-on range in the main clutch operation means may be set to the stroke position where the clutch-engaging hydraulic pressure reaches the maximum hydraulic pressure. Alternatively, it may be set to the stroke position where the clutch-engaging hydraulic pressure is less than the maximum hydraulic pressure.
According to the former case, the clutch for inching with a large volume, which is resistant against frequent use as described above can be used in a whole operating range thereof from a rise of hydraulic oil pressure up to the maximum pressure, thereby securing its durability in frequent inching. However, in this setting of the border, on starting of the vehicle with the main clutch disengaged while a clutch for shifting other than the clutch for inching is engaged, the clutch for inching is completely engaged, and then, an exchange of hydraulic pressure takes place between the clutch for inching and the clutch for shifting corresponding to the determined speed stage. Therefore, during the main clutch operation, another speed stage is formed as if a shifting operation is performed after the speed stage corresponding to the clutch for inching is generated, whereby fluctuations of hydraulic pressure is large.
On the other hand, in the latter case, though a volume of the clutch for inching is not utilized at maximum as an inching application, when a vehicle starting from the main clutch disengaged with a speed range corresponding to a wet type clutch other than the clutch for inching determined, hydraulic pressure does not fluctuate so much and it is possible to transit smoothly from the main-clutch-off state to the determined speed stage set because hydraulic pressure of the wet type clutch corresponding to the determined speed stage rises before the clutch for inching reaches the maximum hydraulic pressure.
Further, a border between the main-clutch-off range and the inching range is set to the stroke position of the main clutch operation means, where the minimum engaging hydraulic pressure required for the clutch for inching rises. In the main-clutch-off range, all the wet type clutches for shifting play till the minimum hydraulic pressure rises in each of the wet type clutches. Accordingly, the drag-preventing clutch apparatus is completely engaged in inching as described above, thereby securing durability of the drag-preventing clutch apparatus. Also, the transition from the main-clutch-off range the inching range becomes smooth so that a spontaneous main clutch operation may be attained.
A second object of the present invention is to configure a vehicle transmission system including a shift mechanism having a plurality of wet type clutches for shifting among which one clutch is selectively engaged to form one speed range, wherein intensive fluctuations of hydraulic pressure associated with the switch of speed stage is not generated so as to establish a smooth shifting.
To achieve this object, according to the present invention, in a shift control mechanism controlling the flow of hydraulic oil for controlling engagement/disengagement of each wet type clutch for shifting, a throttling mechanism is provided in an oil path which is connected to each wet type clutch and allows hydraulic oil to flow therethrough when the wet type clutch is disengaged. Therefore, the hydraulic pressure fluctuates gradually when the wet type clutch for shifting which has been engaged before the switching of speed stage is disengaged.
In addition to this, a delay relief valve is provided to delay a hydraulic pressure rise in an oil path which communicates hydraulic oil to the wet type clutch for shifting selectively engaged after switching. Therefore, the hydraulic oil pressure of both the wet type clutch for shifting to be disengaged and the wet type clutch for shifting to be engaged in association with a shift operation fluctuates gradually and the state of half-clutch is emerged during the transition period in shifting. Accordingly, it is possible to provide a favorable shift mechanism without an impact resulting from fluctuations of hydraulic pressure associated with shifting while using wet type clutches.
These and other objects, structures and effects of the invention will appear in the following detailed description based on the accompanying drawing.